388 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK iv. 



The most recently invented process of ice-making is that devised 

 by M. Pictet, who utilizes sulphurous acid. The following is a 

 description of a machine which can produce 250 kilogrammes of 

 ice per hour : 



A cylindrical tubular copper boiler has a length of 2 metres and 

 a diameter of 35 centimetres ; 150 tubes of 15 millimetres traverse 

 its entire length, and are soldered by their extremities to the two 

 ends. This first boiler is the refrigerator. It is placed horizontally in 

 a large sheet iron vat, which contains 100 tanks of 20 litres each. 

 An incongealable liquid, salted water, is constantly circulating in the 

 interior of the refrigerator by means of a helix. This liquid is re- 

 cooled to about 7 in a normal course, and it is in contact on its return 

 with the sides of the tanks which contain the water to be frozen. 



In the space reserved between the tubes of the refrigerator, the 

 sulphurous acid liquid is volatilised, its vapours are drawn up by an 

 aspirating force-pump, which compresses them without the condenser. 

 This condenser is a tubular boiler, the same as the refrigerator ; only 

 a current of ordinary water passes constantly into the interior of the 

 tubes to carry off' the heat produced by the change of the gaseous 

 into the liquid state of the sulphurous acid, and by the work of com- 

 pression. A tube furnished with a gauge tap, adjusted by the hand 

 once for all, permits the liquefied sulphurous acid to return into the 

 refrigerator to be subjected anew to volatilisation. 



The work necessary to manufacture 250 killogrammes of ice per 

 hour is at the most seven-horse power. 



A cold of 7 in the bath is amply sufficient to obtain in the tanks 

 a rapid and in every way economical congelation. 



With these mechanical arrangements the following important 

 advantages are realised : 1. The pressure never exceeds four atmo- 

 spheres. 2. There is never any entry of air to fear, the pressures, as 

 far as 10 C., being always above that of the atmosphere. 3. The 

 volatile liquid employed is perfectly stable, undecomposable, and 

 without chemical action on metals. 4. All greasing in the machine is 

 dispensed with. 5. The volatile liquid is obtained at a very low price, 

 and it is accompained by no danger of explosion or fire. 6. The cost 

 of production of the ice approaches very near to the theoretic 

 minimum ; it is about 10 francs per ton of ice. 



